The Internet Protocol (IP) multimedia subsystem (IMS) is a standardized architecture for providing multimedia services over any network supporting packet-based communications. IMS can also be extended to circuit-switched networks through appropriate gateways. IMS is configured to provide a centralized service control system across different network architectures. As such, IMS can support multimedia services over any type of access network. These access networks may support fixed or wireless communications, as long as there is a mechanism to support packet-based communications. IMS runs over the standard IP. IMS generally uses VoIP technology based on a third generation partnership project (3GPP) implementation of the Session Initiation Protocol (SIP). With IMS, services can be provided to subscribers irrespective of their location, access technology, and terminal.
With reference to FIG. 1, an exemplary IMS architecture 10 is illustrated. In this example, user elements (UE) 12 are capable of communicating with each other via their respective access networks 14. For a given call or session between the user elements 12, the IMS architecture 10 will provide the requisite signaling to establish and control the call or session. For conciseness and readability, calls or sessions are collectively referred to as sessions. Further, the media delivered in the sessions may be data, audio, video, or voice. The IMS architecture 10 employs several call/session control functions (CSCFs), which are implemented as SIP servers or proxies. These CSCFs are used to process SIP signaling messages, which facilitate the signaling required for establishing and controlling the sessions.
As illustrated, session signaling may be provided in various networks, such as a visited network 18, a home network 16, and a called network 20. The visited network 18 represents the network currently supporting a roaming user element 12 of user A, who is the calling party originating a session. The home network 16 is the home services network for the user element 12 of user A, and the called network 20 is a visited or home network for the user element 12 of user B, who is the called party that is terminating the session.
Each of these networks may include CSCFs. In the IMS architecture 10, the CSCFs are implemented having three primary functions: a proxy CSCF (P-CSCF) 22, an interrogating CSCF (I-CSCF) 24, and a serving CSCF (S-CSCF) 26. The P-CSCF 22 is a SIP proxy that is generally the first point of contact for a user element 12, and can be located in a visited network 18 or home network 16. As illustrated, the P-CSCF 22 in the visited network 18 is associated with user element 12 of user A. User A's user element 12 may be assigned to the P-CSCF 22 in the visited network 18 during registration. The P-CSCF 22 in the visited network 18 is in the signaling path of all signaling messages for the session and will establish a security association with the User A's user element 12. The P-CSCF 22 in the visited network 18 may also compress and decompress SIP messages in an effort to reduce signaling overhead or increase response times over slow radio links. Further, the P-CSCF 22 may map the user ID associated with the user element 12 to an appropriate I-CSCF 24.
To initiate the session with user B's user element 12, user A's user element 12 will send to the P-CSCF 22 in the visited network 18 a session initiation message identifying user B's user element as the called party. The P-CSCF 22 in the visited network will route the call to the I-CSCF 24 in the home network 16. The I-CSCF 24 is also a SIP proxy and is generally located at the edge of an administrative domain, which is the home service network in this example. The IP address of the I-CSCF 24 is published in the domain name service of the administrative domain, such that the P-CSCF 22 in the visited network 18, as well as any other entities, can locate the I-CSCF 24 and use it as a point of entry for all signaling messages for the administrative domain.
Upon receipt of the session initiation message from the P-CSCF 22 in the visited network 18, the I-CSCF 24 may access a home subscriber server (HSS) 28 to identify the S-CSCF 26 to use for session control. The HSS 28 is essentially a master database that supports various network entities that are involved in establishing and controlling sessions. The HSS 28 contains user profiles and other related subscription information, assists in authentication and authorization of a user, and can provide information about the physical location of a user by keeping track of the location of the user element 12.
The S-CSCF 26 is generally the central signaling node in the IMS architecture 10 for the user element 12 of user A. Upon receipt of the session initiation message from the I-CSCF 26 in the home network 16. The S-CSCF 26 in the home network will then route the session initiation message to the I-CSCF 24 in the called network 20, which is serving user B's user element 12. The I-CSCF 24 in the home network 16 will may access another HSS (not shown) to determine the location of user B's user element 12 and identify the S-CSCF 26 in the called network 20 to use for session control. Upon receipt of the session initiation message from the I-CSCF 24 in the called network 20, the S-CSCF 26 in the called network 20 will route the call to the P-CSCF 22, which is serving user B's user element 12 in the called network. The P-CSCF 22 in the called network 20 will then route the session initiation message to the user element 22.
The S-CSCFs 26 used for the routing the session initiation message remain in the signaling path for subsequent signaling messages for the session supported between the user elements 12, and can inspect every signaling message traveling in either direction. Similarly, the P-CSCF's 22 and I-CSCFs 24 invoked during initial routing may remain in the signaling path wherein all signaling messages exchanged between the user element 12 for a session are also passed through these CSCFs. The media session is provided over a transport plane 30 and session control is provided in the control plane 32, which is comprised of the P-CSCF 22, I-CSCF 24, and S-CSCF 26.
Based on inspecting the signaling messages, the S-CSCFs 26 can determine if and when to invoke multimedia services for the user elements 12 or associated sessions. With reference to FIG. 2, the multimedia services are provided in a service plane 34, which is generally made up of numerous application servers 36 capable of providing one or more multimedia services. To provide a multimedia service, the S-CSCF 26 will identify the appropriate multimedia service and forward signaling messages to the application server 36 chosen to provide the multimedia service. The application server 36 will provide the multimedia service by effectively processing the signaling message and returning the processed signaling message back to the S-CSCF 26, if necessary, which will forward the signaling message in a desired fashion. As illustrated, the application server 36 providing a selected multimedia service may also remain in the signaling path.
The evolution of IMS has enabled application servers 36 to initiate sessions directly with a user element 12 or between user elements 12. As illustrated in FIG. 3, an application server 36 may be configured to initiate a video session with user B's user element 12 wherein streaming video is provided to user B's user element 12 from the application server 36. To initiate sessions from an application server 36, developers reconfigured the applications servers 36 and the HSSs 28 to be able to interact with one another, such that the application servers 36 can directly access the HSSs 28 to identify the S-CSCF 26 in the home network 16. Once the application server 36 is aware of the S-CSCF 26 in the home network 16 to use for session control, the session is routed to the S-CSCF 26 in the home network 16. The S-CSCF 26 in the home network 16 will then route the session to the I-CSCF 24 in the called network 20. The I-CSCF 24 will route the session to the S-CSCF 26 in the called network 20. Upon reaching the S-CSCF 26 in the called network 20, the session is routed to user B's user element 12 via the P-CSCF 22 in the called network 20. The application server 36 and user B's user element 12 may exchange messaging over the signaling path to establish the media session.
As noted, the application server 36 may operate in a similar fashion to establish a session between the user elements 12. Regardless of whether the application server 36 provides a session to a user element 12 or initiates and controls a session between user elements 12, the application server 36 has to access the HSS 28 to identify an S-CSCF 26 to invoke for session control. Direct access to the HSS 28 by an application server 36 is undesirable, because the HSS 28 stores critical information regarding the entire IMS network and the nodes and user elements supported thereby. Only secure and authorized application servers 36 can be allowed to access the HSS 28. Generally, application servers 36 are authorized and maintained within the confines of the IMS network. Since one of IMS's goals is to provide new and enhanced services to users, the need for significant security measures has a limiting effect on the proliferation of services and application servers 36 that are available to the IMS users.
For those application servers 36 that are authorized to access the HSSs 28, there is a need for additional protocols simply to communicate with the HSSs 28. Many equipment providers are leery of incorporating the additional protocols in the application servers 36 to facilitate HSS access. The result is a further limiting of the proliferation of services and application servers 36 available to user elements 12.
Accordingly, there is a need for a technique to initiate sessions from application servers 36 without requiring the application servers 36 to interact with the HSSs 28. There is also a need to allow the application servers 36 to initiate such sessions using standard protocols used to interact with S-CSCFs 26. There is a further need to allow non-IMS application servers 36 to gain access to the IMS and initiate sessions in a secure yet efficient fashion.